The addition of molybdenum to steel welds in quite small concentrations leads to a variety of anomalous microstructural and mechanical property effects. In some cases, the effects manifest even when there are no obvious changes in microstructure at the resolution of a transmission electron microscope. There are two particular discrepancies. A quantitative analysis of molybdenum-containing steel welds indicates that there is a degree of strengthening which cannot be explained by the known solid solution or microstructural effects of molybdenum in steels. Secondly, in multirun welds, the addition of molybdenum appears to make the microstructure which evolves during solidification (the primary microstruet1lre) extremely stable. These and other associated phenomena are examined in this thesis. The molybdenum effects mentioned above have been reproduced in detail, using a series of 'high-purity' multirun welds. Having confirmed that molybdenum increases the fraction of primary microstructure in such welds, an attempt was made to see whether the effect is attributable to a change in the austenitisation characteristics with alloying additions. Extensive work using dilatometric techniques backed by microscopy analysis has demonstrated that molybdenum does not lead to any substantial or unexpected changes in the ability to form austenite. The second hypothesis, that the primary microstructure is stabilised as molybdenum increases the tempering resistance, is proven and provides a good explanation of the observations. A series of tempering experiments have established that the anomalously high strength of the molybdenum containing welds cannot be attributed to solid solution strengthening or microstructural effects. Indeed, it appears that there is a submicroscopic secondary hardening type effect which enhances the strength. Even the thermal treatment that occurs as the weld cools from the solidification temperature is shown to be sufficient to induce molybdenum based secondary hardening type effects. Some preliminary atomic resolution effects also lend support to this concept. Titanium as a trace element is important in steel welds, as an element which promotes the intragranular nucleation of acicular ferrite on titanium-rich phases. It is demonstrated that the titanium effect is not intrinsically different for molybdenumcontaining welds. However, the extra hardenability associated with molybdenum certainly helps to suppress the formation of other grain boundary nucleated phases which might swamp events that occur on the inclusions within the grains. An interesting observation is that titanium has a positive effect in limiting the grain boundary nucleated phases, because the intragranularly nucleated acicular ferrite to some extent stifles the formation of other phases.